Among the most important technical problems facing the designer of effective systems using laser radiation for recording information on 3D media, particularly on multilayer optical discs, is the one of positioning the laser beam in the recording area inside the 3D medium so that a change in the optical properties of the recording medium as a result of storage of information is precisely localized in space. The change in the optical properties in this case may be defined as a change in the refraction index, absorption index, scattering coefficient or other optical, e.g. fluorescent, properties of the medium. The space occupied by recording one bit of data (pixel) must be minimized thus increasing the data recording density and volume for storage of data on one medium. The minimum lateral dimension of a pixel is limited to a value of about half the wavelength of the recording radiation. This directly follows from the diffraction theory and so is a fundamental physical limitation. By recording data with use of short-wave optical radiation and short-focus optical systems with large numerical apertures and corrected spherical aberration which focus this radiation within the recording medium one can obtain a high volume density of the recorded information.
Known in the art is a 3D device for recording and reading information (Kawata Y., Nakano M., Lee S-C. Three-dimensional optical data storage using three-dimensional optics.—Optical Engineering, vol. 40 (10), p. 2247-2254). This paper describes various (one-photon and two-photon) media and various versions of a device for recording, erasing and reading information, based on the principle of confocal microscopy. This device records information by focusing the radiation using the recording wavelength within the material and changing the refraction index of said material by exposing it to the radiation. The recorded data is then read out by registering the regions with the changed refraction index by the value of phase distortion of the reading light beam with a wavelength other than that of the recording radiation. The authors of the above paper point out the disadvantages of the proposed means and devices. The cross-talk between the layers is great where one-photon media are used, while the use of two-photon media requires a more powerful and shorter recording laser pulse, which makes the miniaturization of the laser source impossible. Besides, since the method of measurement of phase disturbances is highly sensitive, the data recording requires media with very high optical homogeneity of material and optical surface quality.
Also known in the art is a device for recording, erasing and reading the data within a photochromic material, typically spirbenzopyran, maintained in a 3D matrix, typically of polymer (U.S. Pat. No. 5,268,862. Three-dimensional optical memory, publ. Dec. 7, 1993). The device comprises laser radiation sources and systems for optical positioning and focusing of the laser beams. The material used for recording has two stable forms, spiropyran and merocyanine. Transition from the first form to the second is performed through a two-photon absorption occurring on the wavelength of 532 nm. The recording medium is illuminated by two focused laser beams with the specified wavelength in two mutually perpendicular directions. In this manner the spatial positioning of the area of interaction of two laser beams in a 3D space is achieved, while the conversion of the first form of the photochromic material into the other only occurs where the focal regions of the beams intersect. The second form of the photochromic material exhibits fluorescence when exposed to irradiation with light on a wavelength of 1,064 nm. As the material is irradiated, one can read the information by using the said fluorescence. The information recorded on a 3D material can be erased by heating the medium, total or local, say, by irradiating it with light having a wavelength of 2.12 μm. The disadvantages of this device are mostly analogous to those of the one described above. Inasmuch as the process of conversion of the recording material from one form to the other is a two-photon one, it is necessary to use radiation sources having a super-high peak power. The need for positioning the point of intersection of the focal regions of two orthogonal beams within the volume of the material limits the degree of decreasing the volume of recording one bit of information by the given device to units and tens of micrometers, while potentially the light beam can be focused onto an area with a lateral dimension less than micron. Moreover, the degree of optical homogeneity of the medium and quality of the surfaces limiting the volume with the photochromic material must be very high. Where polymers are used as a binding matrix, obtaining the required optical quality under the conditions of serial production of 3D photochromic materials is quite problematic.
Also known in the art is device for recording/reading optical information on a multilayer recording medium (U.S. Pat. No. 7,345,967. Optical pickup unit. Publ. Mar. 18, 2008), comprising a source of radiation, a beam splitter, a controlled spherical aberration corrector and an objective lens as well as an optical sensor (photo receiver) optically coupled with the objective lens via the beam splitter, all arranged in series in the direction of the beam. The given device records and reads information by radiation on one wavelength. The operating modes of the device are selected by changing the power of the radiation aimed at the radiation recording medium. The main disadvantage of this device resides in an inevitable danger of losing the recorded information during readout. To mitigate it in the prior art device, it is suggested to decrease the radiation source power during information read-out down to the minimum permissible values at which the useful signal only slightly exceeds the noise level.
The problem of losing information during its read-out is particularly acute where materials with one-photon mechanisms of interaction of light with substance are used as the 3D recording media. In real light fluxes required for reading information using the changes in the optical density of the medium the information is erased within 5-10 reading cycles. To solve the given problem, some authors propose the method of reading information by using radiation on a wavelength located at the optical absorption edge of the photochromic material (see Satoshi Kawata, Yoshimasa Kawata. Three-dimensional optical data storage using photochromic materials. Chem. Rev. 2000, 100, 1777-1788). In this method, the volume of a pixel increases in real practice ten- or even hundred-fold. Besides, where one-photon media are used practically all authors also point out a slight crosstalk between the layers carrying the recorded information.
Considering the above, the most justified option to choose in this case seems to be the one involving recording/erasing information using the threshold two-photon media in which information is recorded and erased only when a certain threshold intensity of light has been reached. However, such media require very high radiation power for reading and erasing information, while the practical embodiment of such power at present in miniaturized devices is impossible.
From the viewpoint of practical embodiment, the simplest system for reading/erasing information in 3D media is represented by a device comprising two sources of radiation with different wavelengths, optically coupled with one focusing system having the means for controlling the position of the focusing region within the 3D medium, a spherical aberration correction unit as well as a receiver of optical radiation emitted by the 3D medium during read-out of the information recorded on it (U.S. Pat. No. 7,436,750. Optical storage with ultrahigh storage capacity, publ. Oct. 14, 2008). In this device, while information is recorded, a radiation having wavelength λ1 is focused onto the recording medium consisting of alternating layers of transparent and photochromic materials. When exposed to said radiation, the photochromic material in the selected recording layer changes its optical properties and develops a capacity, when exposed to a radiation having wavelength λ2, to fluoresce on wavelength λ3. As the information is being read out, the radiation having wavelength λ2 is focused onto the recording medium. The fluorescent light emitted by the photochromic material on wavelength λ3 within the limits of the pixels previously exposed to the light with wavelength λ1 and containing bits of information is registered by an optical sensor (photo receiver). This system illustrates, more than any other, the technical essence of the claimed device and so is accepted by the authors as the prototype.
The main disadvantage of the prototype resides in the low density of the recorded information, which is due to the cross-talk between the layers of the photochromic material. The cross-talk occurs as a result of the fact that when information is recorded onto a deep-lying signal layer of the photochromic material the radiation with wavelength λ1 passes through the higher-positioned photochromic layers thus inevitably inducing in the latter the same processes as those occurring in the signal layer while the information is recorded. To decrease the crosstalk between the photochromic layers, it is necessary to either reduce their number, or increase the thickness of the layers of the transparent material between them. Neither solution of the problem is optimal and results in either limiting the maximum volume of data that can be recorded on one data medium or increasing the thickness of the multilayer optical disc and thus reducing the recording density. There are two methods used to suppress the cross-talk between the layers in practical embodiment of the prototype: the first method uses, as a source of recording radiation, a powerful laser which initiates the two-photon data recording process in the signal photochromic layer, and the other has an optical layout to register fluorescent light, which comprises a chromatic aberration compensation element, short-focus objective lens and an aperture diaphragm of several microns, all arranged in series thus forming a confocal train for registering the fluorescence radiation. The use of the confocal registration layout markedly reduces the value of the useful fluorescent signal, so a photomultiplier is proposed to be used in the prototype as a fluorescent light detector. On the one hand, the practical embodiment described in the prototype confirms existence of a serious problem with the cross-talk between the layers, while on the other hand the state-of-the-art technology makes impossible application of the prototype in the production of commercial information recording devices based on multilayer optical discs because of the high cost and complex design of the device.